Accelerators for epoxide curing agents containing cyanamide

ABSTRACT

Accelerators for epoxide curing agents containing cyanamide are described which use an adduct and/or reaction product of (a) a specific imidazole derivative with (b) an organic acid having a pK A  value of 0.5 to 5.0. 
     The accelerators according to the invention impart to the epoxy resin-curing agent-accelerator system a longer shelf life compared with known accelerators.

DESCRIPTION

The present invention relates to accelerators for epoxide curing agentscontaining cyanamide.

It is known that cyanamide may be used as curing agent for epoxy resincompounds. In this connection cyanamide may be used in anhydrous form(cf. DE-OS 2,122,955 and DE-OS 2,166,606) or an aqueous solution, in thepresence or absence of urea according to DE-OS 2,743,015. According toDE-OS 2,459,752, the curing agent based on cyanamide may also containspecial stabilizers in the form of carboxylic acids. According to DE-OS2,932,785 it is further possible to use substituted cyanamides oforganic primary amines instead of cyanamide, in which case aliphatic,cycloaliphatic, aromatic or heterocyclic radicals are introduced in thecuring agent system. The preparation of these substituted cyanamides is,however, technically exacting and hence costly. Finally, DE-OS 3,510,952proposes to use cyanamide or a mixture of cyanamide and dicyandiamide inan organic solvent. All these publications recommend tertiary amines,for example benzyldimethylamine, or imidazoles as accelerators.

The solutions of curing agents based on cyanamide and the mixturesconsisting of a liquid epoxy resin or an epoxy resin solution and solidcyanamide or a cyanamide solution are usually stable for a few days. If,however, an accelerator corresponding to the prior art, which isnecessary for rapid hardening, is added to these mixtures, the systemepoxy resin-cyanamide-accelerator becomes unstable, i.e. a rapidincrease in viscosity occurs. At higher temperatures, especially atabout 40° C., it is even possible for a strong exothermicself-accelerating reaction to set in. This low shelf life of the epoxyresin-curing agent-accelerator system very seriously restricts practicalapplication of cyanamide as epoxide curing agent.

The object forming the basis of the present invention was to developaccelerators for epoxide curing agents containing cyanamide which do nothave the said shortcomings of the prior art but possess an increasedshelf life of the epoxy resin-curing agent-accelerator system.

According to the invention this object is achieved by using asaccelerator an adduct and/or a reaction product of (a) an imidazole ofthe formula ##STR1## in which R₁, R₂, R₃ and R₄ are hydrogen, alkyl of 1to 3 carbon atoms or phenyl with (b) an organic acid having a pK_(A)value of 0.5 to 5.0. For it has been found, surprisingly, that theaccelerators according to the invention keep the viscosity of the epoxyresin mixture constant over a relatively long period of time.

The accelerators according to the present invention consist of an adductand/or a reaction product of a given imidazole derivative and an organicacid. Within the scope of the present invention the term "adduct" isunderstood to mean loosely linked addition products, while the term"reaction products" refers especially to salts and/or amides of theparticular imidazole derivative with the corresponding carboxylic acids.

The imidazole derivatives which can be used according to the invention,may be wholly unsubstituted or substituted in the 1-, 2-, 4- and/or5-position. Suitable substituents for this purpose are especially alkylradicals of 1 to 3 carbon atoms in the main chain as well as phenylradicals. The alkyl radicals may be saturated, for example methyl, ethylor n-propyl radicals, or they may be unsaturated, that is, alkenylradicals such as vinyl or allyl radicals, but they may also be branched,for example isopropyl radicals.

In their turn the alkyl radicals and phenyl radicals of the substitutedimidazoles may be also substituted, suitable substituents for the alkylradicals being CN, OH, OCH₃ as well as phenyl or aromatic substituents.Examples of such substituted alkyl radicals are 2-cyanoethyl,2-hydroxyethyl, 2-methoxyethyl, benzyl or2-(3,5-diamino-s-triazinyl)ethyl radicals.

The phenyl radicals in the 1-, 2-, 4- or 5-position of the imidazole maybe likewise substituted, suitable substituents being OH, OCH₃, NH₂,halogen or CN groups.

The imidazoles which may be used according to the invention, are knownand some are available commercially, or they may be very simply preparedby conventional methods.

It is an essential part of the invention that the adducts and/orreaction products of the imidazole derivative should be produced usingsuch organic acids whose pK_(A) value lies between 0.5 and 5.0. Examplesof such acids are acetic acid, formic acid, maleic acid, fumaric acid,lactic acid, salicylic acid, p-hydroxybenzoic acid and anthranilic acid.Salicylic acid is particularly preferred.

Within the scope of the present invention it is of course also possibleto use instead of the organic acids their salts and/or their acidanhydrides. Of the acid anhydrides, maleic anhydride has proved to beparticularly advantageous for this purpose.

The molar ratio of the two components imidazole derivative and organicacid may be varied within a wide range. Preferred molar ratios ofimidazole to acid are 1:1 to 1:2.5, preferably 1:1 to 1:1.2. When acidanhydrides are used, attention must be paid to the twin acid functioni.e. 50% of the molar amount compared with the acid is used.

The preparation of the accelerators according to the invention from thecomponents (a) and (b) may be carried out in a variety of ways, thevariants I to III below being particularly technically simple and costeffective.

Variant I: Intimate mixing of the components in the solid state,followed by grinding if appropriate.

Variant II: Mixing of the components, heating to melting and, ifappropriate, comminuting after cooling.

Variant III: Dissolving the components in suitable solvents such aswater, alcohols (for example methanol or ethanol), dimethylformamide,ketones etc., on heating and crystallizing out the product (for exampleas the salts) on cooling.

The accelerators according to the invention may be employed withcyanamide for the curing of conventional epoxy resin systems without anydifficulties; the epoxy resins may be present both in liquid anddissolved form.

The amount of the accelerator used according to the invention isgoverned essentially by the amount of the curing agent or the epoxyresin. The amounts of the accelerator used are preferably 1 to 10% byweight based on the weight of the cyanamide and usually 0.1 to 1% byweight based on the weight of the epoxy resin.

If the accelerators according to the invention are incorporated in theepoxy resins in solid form, then they should be in a form as finelydivided as possible in order to achieve a distribution as uniform aspossible and optimum effect. The particle size should accordingly be<150 μm.

The epoxide curing with the aid of the accelerators according to theinvention is carried out at temperatures of 100° to 200° C., preferablyof 140° to 180° C. In the curing process it is possible to make use ofthe heat of the exothermic reaction to attain complete curing even atlow starting temperatures.

The accelerators according to the invention provide all the benefitswhich are essential for the practical technical application, such assatisfactory action, problem-free handling and good shelf life of theepoxy resin-curing agent-accelerator mixtures.

The examples below are intended to elucidate the invention withoutlimiting it in any way.

EXAMPLES

The viscosity characteristics of epoxy resin mixtures containingcyanamide are investigated in the Examples 1 and 2 below using differentaccelerators.

EXAMPLE 1

A 40% solution of cyanamide in methoxypropanol was used as the curingagent solution.

The following accelerators were used:

Accelerator 1=benzyldimethylamine (comparison)

Accelerator 2=2-methylimidazole (comparison)

Accelerator 3=adduct of 2-methylimidazole and salicylic acid (molarratio 1:1.04) (prepared by variant I by intimately mixing the componentswith a particle size <100 μm)

Accelerator 4=adduct of 2-methylimidazole and maleic anhydride (molarratio 1:0.52) (produced by variant I by intimately mixing thecomponents, followed by grinding with cooling to a particle size of <100μm)

To study the viscosity characteristics, the viscosities of the abovemixtures were measured at 25° C. using a Ubbelohde viscometerimmediately after the preparation and then at intervals of 24 hoursduring storage at 22° C. The viscosity increases relative to the initialviscosity are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Mixture of 133.3 g of epoxy resin (Rutapox 2040                               MEK 75), 7.5 g of curing agent solution, 5 g of                               methoxypropanol and the stated amounts of accelerator.                                           Viscosity increase                                                     Initial                                                                              based on initial                                                       viscosity                                                                            viscosity                                                  Accelerator                                                                            Amount   (cSt)    1 d     2 d  3 d                                   ______________________________________                                        No. 1 (C)                                                                              0.1 g    484.8    121%    --   --                                    No. 2 (C)                                                                              0.1 g    475.4    16%     95%  236%                                  No. 3 (I)                                                                              0.2 g    468.7    5%      11%  16%                                   No. 4 (I)                                                                              0.2 g    486.3    3%      13%  21%                                   ______________________________________                                         (C) = comparison                                                              (I) = invention                                                          

EXAMPLE 2

A solution of 5% by weight of dicyandiamide and 5% by weight ofcyanamide in methoxypropanol were used as the curing agent solution.

The products corresponding to Example 1 were used as accelerators 1 to4.

For the viscosity studies corresponding to Example 1 the relativeviscosity increases listed in Table 2 were measured.

                  TABLE 2                                                         ______________________________________                                        Mixture of 133.3 g of epoxy resin (Rutapox 2040                               MEK 75), 30 g of curing agent solution, 5 g of methoxy-                       propanol and the stated amounts of accelerator.                                                   Viscosity increase                                                     Initial                                                                              based on initial                                                       viscosity                                                                            viscosity                                                 Accelerator                                                                            Amount    (cSt)    1 d    2 d   3 d                                  ______________________________________                                        No. 1 (C)                                                                              0.1 g     139.1    32%    55%   85%                                  No. 2 (C)                                                                              0.1 g     137.9    8%     23%   51%                                  No. 3 (I)                                                                              0.2 g     134.7    1%      8%    8%                                  No. 4 (I)                                                                              0.2 g     138.9    4%      8%   16%                                  ______________________________________                                         (C) = comparison                                                              (I) = invention                                                          

I claim:
 1. A process for accelerating the curing of epoxide resinsusing a cyanamide curing agent which comprises using as an acceleratoran adduct and/or a reactant product of (a) an imidazole of the formula##STR2## in which R₁, R₂, R₃, and R₄ are hydrogen, alkyl or alkenyl of 1to 3 carbon atoms or phenyl with (b) an organic acid, organic acid saltor organic acid anhydride having a pK_(A) value of 0.5 to 5.0.
 2. Theprocess as claimed in claim 1, wherein at least one radical of theimidazole is alkenyl.
 3. The process as claimed in claim 1, wherein thealkyl and/or phenyl radical is substituted.
 4. The process as claimed inclaim 3, wherein the substituent(s) for the alkyl radical are CN, OH,OCH₃, phenyl or 3,5-diamino-s-triazinyl.
 5. The process as claimed inclaim 3, wherein the substituent(s) for the phenyl radical are OH, OCH₃,NH₂, halogen or CN.
 6. The process as claimed in claim 1, whereinsalicylic acid is used as the organic acid.
 7. The process as claimed inclaim 1, wherein organic acid salts and/or acid anhydrides are used. 8.The process as claimed in claim 7, wherein maleic anhydride is used asthe acid anhydride.
 9. The process as claimed in claim 1, wherein about1 to about 10% by weight of said accelerator is used based on the weightof the cyanamide curing agent.
 10. The process as claimed in claim 1,wherein said accelerator is particulate and has a particle size of lessthan 150 μm.
 11. The process as claimed in claim 1, wherein said curingis performed at temperatures of 100° to 200° C.